A standard motor-assist door latch is described in U.S. Pat. No. 5,394,718 of Hotzl as having a main lock housing, a bolt movable in the housing between a locked position projecting from the housing and an unlocked position largely recessed in the housing, a key-operable mechanism having an actuator movable by turning of an appropriately bitted key in the mechanism, and a linkage between the actuator and the bolt for displacing the bolt by means of the actuator. A secondary housing is provided adjacent the lock housing and an actuator element coupled and movable with the linkage extends from the lock housing to the secondary housing. The element moves in one direction relative to the secondary housing on movement of the bolt from the locked to the unlocked position and in the opposite direction on movement of the bolt from the unlocked to the locked position. An electric motor on the secondary housing connected to the element is energizable for displacing same in both directions and thereby also displacing the bolt between its positions. Switches on the secondary housing juxtaposed with the element and connected to the motor detect movement of the element in either direction when actuated through the linkage by the key and energize the motor to move the element in the same direction it is already moving in. Thus the motor gives the key a sort of power assist.
Such an assembly is typically used to operate a high-security multibolt door lock which presses bolts in several directions into the jamb. Thus the motor is normally needed to move the considerable amount of equipment that must be displaced to lock and unlock the door. Even if all the various bolts and linkages are finely machined and oiled, they nonetheless constitute quite some mass that would make turning the key a serious chore and, therefore, would militate against use of the lock.
The obvious problem with this type of motor-assisted lock is that if the power fails the door is locked and cannot be opened. It becomes necessary to provide some sort of auxiliary unlocking mechanism that not only unnecessarily augments the cost of the lock, but presents a weak point at which the lock can be attacked for unauthorized entry.